Guoqiang Qin

The improvement of densification and fracture toughness in high-entropy ceramics is important to realizing their practical applications. In this study, SiC whiskers and metal Ni additions were incorporated to solve these problems of high-entropy boride ceramics. The influence of sintering temperatures (1450–1650 °C) on the densification, microstructure, hardness, fracture toughness, and bending strength of (M_0.2Ti_0.2Ta_0.2V_0.2Nb_0.2)B₂-SiC_w-Ni (M=Hf, Zr, or Cr) composites prepared by hot-pressing technology were studied. Results showed that when SiC whiskers and metal Ni additions were used as additives, increasing sintering temperatures from 1450 to 1600 °C promoted the densification of high-entropy boride ceramics. This was mainly attributed to the high sintering driving force. However, when the temperature further increased to 1650 °C, their densification behavior decreased. At a sintering temperature of 1600 °C, these high-entropy borides ceramics all had the highest densification behavior, leading to their high hardness and fracture toughness. The highest relative density was 96.3%, the highest hardness was 22.02 GPa, and the highest fracture toughness was 13.25 MPa·m^1/2, which was improved by the co-function of SiC whiskers and plastic metal Ni. Meanwhile, in the adopted sintering temperature range of 1450 to 1650 °C, the highest bending strength at room temperature of these high-entropy boride ceramics could reach 320.8 MPa. Therefore, this research offers an effective densification, strengthening, and toughening method for high-entropy boride composites at a low sintering temperature.

This research article presents a comprehensive first-principles study on the piezoelectric properties of Wurtzite Aluminum Nitride (AlN) films under in-plane strain conditions. By calculating the piezoelectric tensor coefficients (e33, e31, and e15), we investigate the variation patterns of these constants with respect to in-plane strain. Our results indicate significant changes in the piezoelectric constants within the range of in-plane strain considered, exhibiting a linear trend despite opposite trends for e33 compared to e31 and e15. This study highlights the extreme sensitivity of AlN films’ piezoelectric performance to in-plane strain, suggesting its potential as an effective means for tuning and optimizing the piezoelectric properties of AlN-based devices.

A metamaterial structure using steel reinforcement and dielectric cubes was designed, which was introduced into concrete to improve the electromagnetic wave absorption performance of concrete. A near-unit absorption peak at 9.82 GHz was generated in simulation and experimental results. It is also demonstrated that this metamaterial absorbing concrete is insensitive to polarization and can operate at wide angle of incidence.

Ternary wurtzite In_0.5Al_0.5N films and coatings are promising candidates for microelectronic or optoelectronic devices due to their excellent physical and chemical properties. However, as a universal and non-negligible phenomenon, in-plane strain and its effects on the structure and properties of In_0.5Al_0.5N still need systematic research. In particular, the deformation mechanism of In_0.5Al_0.5N under biaxial strain is not clearly understood currently. To reveal the role of the internal relaxation effect in lattice deformation, the lattice variation, thermal stability, and the electronic properties of ternary wurtzite compound In_0.5Al_0.5N under different biaxial strains are systematically investigated, using first-principles calculations based on density functional theory. The results indicate that, compared with the classic elastic deformation mechanism with constrained atomic coordinates, atom relaxation results in a much smaller Poisson ratio. Moreover, the plastic relaxation In_0.5Al_0.5N phase, generated by free atom relaxation, exhibits higher thermal stability than the elastic relaxation phase, so it is the most likely phase in reality when biaxial strain is imposed. Meanwhile, the biaxial strain has a remarkable influence on the electronic structure of In_0.5Al_0.5N films, where a non-linear variety of energy band gaps can be seen between the valance band and conduction band.

The stability, geometry, microstructure, and specie combination together with the electronic states of the anatase TiO₂ with oxygen defect content of 0, 3.125, 6.25, and 12.5 have been intensively studied within the framework of the density functional theory method. The results show that the TiO₂ with an oxygen defect is not as stable as intrinsic TiO₂. The compound formation enthalpy E_f and the oxygen defect formation energy value tend to be larger for a higher defect content, and the oxygen defect gets harder to be formed. The bonds within the TiO₆ polyhedron are different and not geometrically symmetrical. The bond strengths show distinct diversity, and the primitive cell of anatase TiO₂ show spatial expansion when there are oxygen defects. All bands moved down to the low energy region, and two impurity energy band levels emerged for the anatase TiO₂ with oxygen defect. The energy band gap is decreased from 3.085eV to 1.165eV, 1.0015eV, and 0.43eV. There are generally 7 peaks for the spin density of states function, corresponding to their 5 main bands. For the anatase TiO₂ with an oxygen defect content of 12.5, the spin density of states functions are not horizontal ordinate symmetrical near 1.12eV and 0.31eV. They are formed by oxygen defect energy levels, which is the result of the Ti d and O p state electron polarization. Transitions from weak paramagnetic to antiferromagnetic are found for the anatase TiO₂ with oxygen defect.